1. Technical Field of the Invention
The present invention relates to clock synchronization, and more particularly, to clock synchronization between transmitters and receivers within a multi-channel baud-rate timing recovery system
2. Description of the Related Art
The process of baud-rate timing recovery involves determining a frequency and phase of incoming signals through the use of samples (normally provided by an A/D converter) acquired at the same rate as the incoming data is transmitted. Traditional single channel baud-rate timing recovery systems use a phase detector which drives a loop filter as shown in FIG. 1. The incoming signal 10 is provided to an A/D converter 15, which generates samples of the input signal 10 that are provided to a phase detector 20. The phase detector operates in conjunction with other signal processing elements 22 such as Feed-forward and Decision-Feedback Equalizers and a data slicer in order to provide a phase error signal. The phase error signal is forwarded from the phase detector 20 to a loop filter 25. The output of the loop filter 25 is used to control the output of a voltage-controlled oscillator 30. The output signal of the voltage-controlled oscillator 30 is used as a clock signal to control the sample rate of the A/D converter 15. The goal of the timing recovery loop is to lock on to a remote signal and through acquiring the frequency and phase of the remote signal provide baud-rate A/D converter samples at the optimum sampling period.
In a loop-timed system such as that described above, there exists a master and a slave device as shown in FIG. 2. The master provides the reference timing for the system and the slave must synchronize itself to the master's frequency and phase on one or a plurality of channels. In the case of duplex systems, such as those described by the IEEE 802.3ab 1000Base-T standard, the slave must transmit back to the Master using the acquired timing on one or a plurality of channels. A typical master or slave system will have ECHO cancellers which mitigate the effects of the local transmit signal on the local receive signal. Multi-channel master or slave systems will additionally have Near End Crosstalk (NEXT) Cancellers and Far End Crosstalk (FEXT) Cancellers which mitigate the effects of adjacent transmitters on the local receivers.
In a single channel baud type slave system, the synchronization of the local Receive (Rx) and Transmit (Tx) clock is required so that frequency synchronization (i.e., loop timing) is achieved between the master and slave. The synchronization of the Receive and Transmit clocks also mitigates the problem of ECHO/NEXT canceller misadjustment. This canceller misadjustment occurs when the Rx and Tx clock phases change relative to each other requiring the canceller taps to be readapted. Canceller misadjustment causes a short term increase in the noise of the receiver system and hence results in a poorer quality of data reception.
In a multi-channel slave system, the issue of Receive and Transmit clock synchronization is much more difficult. Each slave Receive/Transmit pair could be synchronized to each other (i.e., TXCLK1=RSCLK1, TXCLK2=RXCLK2, etc.), or an alternative method can be devised wherein all Transmit clocks are synchronous and are in turn synchronized with the frequency of the Receive clocks. The IEEE 802.3ab specification dictates that the latter of these two methods be used in a Phy receiver. This specification creates difficulties for baud-rate timing recovery systems as the synchronization of the four Transmit clocks to each other rather than to each Receive channel causes ECHO/NEXT canceller misadjustment Thus, an improved method of synchronization within multi-channel slave systems would be desirable.